Mutations in the chloride-bicarbonate exchanger gene AE1 cause autosomal dominant but not autosomal recessive distal renal tubular acidosis

Impaired trafficking of human kidney anion exchanger (kAE1) caused by hetero-oligomer formation with a truncated mutant associated with distal renal tubular acidosis

Autosomal dominant distal renal tubular acidosis (dRTA) has been associated with several mutations in the anion exchanger AE1 gene. The effect of an 11-amino-acid C-terminal dRTA truncation mutation (901 stop) on the expression of kidney AE1 (kAE1) and erythroid AE1 was examined in transiently transfected HEK-293 cells. Unlike the wild-type proteins, kAE1 901 stop and AE1 901 stop mutants exhibited impaired trafficking from the endoplasmic reticulum to the plasma membrane as determined by immunolocalization, cell-surface biotinylation, oligosaccharide processing and pulse-chase experiments. The 901 stop mutants were able to bind to an inhibitor affinity resin, suggesting that these mutant membrane proteins were not grossly misfolded. Co-expression of wild-type and mutant kAE1 or AE1 resulted in intracellular retention of the wild-type proteins in a pre-medial Golgi compartment. This dominant negative effect was due to hetero-oligomer formation of the mutant and wild-type proteins. Intracellular retention of kAE1 in the alpha-intercalated cells of the kidney would account for the impaired acid secretion into the urine characteristic of dRTA.

Regulation of AE2-mediated Cl- transport by intracellular or by extracellular pH requires highly conserved amino acid residues of the AE2 NH2-terminal cytoplasmic domain

We reported recently that regulation by intracellular pH (pH(i)) of the murine Cl-/HCO(3)(-) exchanger AE2 requires amino acid residues 310-347 of the polypeptide's NH(2)-terminal cytoplasmic domain. We have now identified individual amino acid residues within this region whose integrity is required for regulation of AE2 by pH. 36Cl- efflux from AE2-expressing Xenopus oocytes was monitored during variation of extracellular pH (pH(o)) with unclamped or clamped pH(i), or during variation of pH(i) at constant pH(o). Wild-type AE2-mediated 36Cl- efflux was profoundly inhibited by acid pH(o), with a value of pH(o50) = 6.87 +/- 0.05, and was stimulated up to 10-fold by the intracellular alkalinization produced by bath removal of the preequilibrated weak acid, butyrate. Systematic hexa-alanine [(A)6]bloc substitutions between aa 312-347 identified the greatest acid shift in pH(o(50)) value, approximately 0.8 pH units in the mutant (A)6 342-347, but only a modest acid-shift in the mutant (A)6 336-341. Two of the six (A)6 mutants retained normal pH(i) sensitivity of 36Cl- efflux, whereas the (A)6 mutants 318-323, 336-341, and 342-347 were not stimulated by intracellular alkalinization. We further evaluated the highly conserved region between aa 336-347 by alanine scan and other mutagenesis of single residues. Significant changes in AE2 sensitivity to pH(o) and to pH(i) were found independently and in concert. The E346A mutation acid-shifted the pH(o(0) value to the same extent whether pH(i) was unclamped or held constant during variation of pH(o). Alanine substitution of the corresponding glutamate residues in the cytoplasmic domains of related AE anion exchanger polypeptides confirmed the general importance of these residues in regulation of anion exchange by pH. Conserved, individual amino acid residues of the AE2 cytoplasmic domain contribute to independent regulation of anion exchange activity by pH(o) as well as pH(i).

Band 3 mutations, distal renal tubular acidosis, and Southeast Asian ovalocytosis

Familial distal renal tubular acidosis (dRTA) and Southeast Asian ovalocytosis (SAO) may coexist in the same patient. Both can originate in mutations of the anion-exchanger 1 gene (AE1), which codes for band 3, the bicarbonate/chloride exchanger in both the red cell membrane and the basolateral membrane of the collecting tubule alpha-intercalated cell. Dominant dRTA is usually due to a mutation of the AE1 gene, which does not alter red cell morphology. SAO is caused by an AE1 mutation that leads to a nine amino acid deletion of red cell band 3, but by itself does not cause dRTA. Recent gene studies have shown that AE1 mutations are responsible for autosomal recessive dRTA in several countries in Southeast Asia; these patients may be homozygous for the mutation or be compound heterozygotes of two different AE1 mutations, one of which is usually the SAO mutation.

Autosomal recessive distal renal tubular acidosis caused by G701D mutation of anion exchanger 1 gene

Anion exchanger 1 (AE1 or band 3), encoded by the AE1 or SLC4A1 gene, regulates chloride-bicarbonate exchange in erythrocytes and alpha-intercalated cells of the distal nephron. Defects of AE1 at the basolateral membrane of alpha-intercalated cells may result in the failure of hydrogen ion secretion at the apical membrane, leading to distal renal tubular acidosis (dRTA). Abnormalities of the AE1 gene were previously reported to be associated with autosomal dominant dRTA. However, recent studies of Thai dRTA families have shown that mutations in this gene result in autosomal recessive (AR) dRTA, giving rise to the postulation that AE1 gene mutations causing AR dRTA might be found commonly in Thai pediatric patients with dRTA. We performed a study of the AE1 gene using DNA linkage, polymerase chain reaction single-strand conformation polymorphism, restriction endonuclease HpaII digestion, and DNA sequence analyses in eight families involving 12 Thai children with dRTA, shown by abnormal urinary acidification using a short acid-loading test, as well as among their family members. Seven patients with dRTA from five families had the same homozygous missense G701D mutation of the AE1 gene. Their parents or siblings heterozygous for the AE1 G701D mutation were clinically normal and did not have abnormal urinary acidification, although a heterozygous sibling in one family had abnormal urinary acidification. Results of this and previous studies show that a homozygous AE1 G701D mutation causes AR dRTA and is a common molecular defect among Thai pediatric patients with dRTA.

Secretory-defect distal renal tubular acidosis is associated with transporter defect in H(+)-ATPase and anion exchanger-1

Recent progress in molecular physiology has permitted us to understand pathophysiology of various channelopathies at a molecular level. The secretion of H(+) from alpha-intercalated cells is mediated by apical plasma membrane H(+)-ATPase and basolateral plasma membrane anion exchanger-1 (AE1). Studies have demonstrated the lack of H(+)-ATPase immunostaining in the intercalated cells in a few patients with distal renal tubular acidosis (dRTA). Mutations in H(+)-ATPase and AE1 gene have recently been reported to cause dRTA. This study extends the investigation of the role of transporter defect in dRTA by using immunohistochemical methods. Eleven patients with hyperchloremic metabolic acidosis were diagnosed functionally to have secretory-defect dRTA: urine pH >5.5 during acidemia, normokalemia or hypokalemia, and urine-to-blood pCO(2) <25 mmHg during bicarbonaturia. Renal biopsy tissue was obtained from each patient, and immunohistochemistry was carried out using antibodies to H(+)-ATPase and AE1. For comparison, renal tissues from the patients who had no evidences of distal acidification defect by functional studies were used: four with glomerulopathy or tubulointerstitial nephritis (disease controls) and three from nephrectomized kidneys for renal cell carcinoma (normal controls). The H(+)-ATPase immunoreactivity in alpha-intercalated cells was almost absent in all of the 11 patients with secretory-defect dRTA. In addition, 7 of 11 patients with secretory-defect dRTA were accompanied by negative AE1 immunoreactivity. In both disease controls and normal controls, the immunoreactivity of H(+)-ATPase and AE1 was strong in alpha-intercalated cells. In conclusion, significant defect in acid-base transporters is the major cause of secretory-defect dRTA.

Genetic diseases of acid-base transporters

Genetic disorders of acid-base transporters involve plasmalemmal and organellar transporters of H(+), HCO3(-), and Cl(-). Autosomal-dominant and -recessive forms of distal renal tubular acidosis (dRTA) are caused by mutations in ion transporters of the acid-secreting Type A intercalated cell of the renal collecting duct. These include the AE1 Cl(-)/HCO3(-) exchanger of the basolateral membrane and at least two subunits of the apical membrane vacuolar (v)H(+)-ATPase, the V1 subunit B1 (associated with deafness) and the V0 subunit a4. Recessive proximal RTA with ocular disease arises from mutations in the electrogenic Na(+)-bicarbonate cotransporter NBC1 of the proximal tubular cell basolateral membrane. Recessive mixed proximal-distal RTA accompanied by osteopetrosis and mental retardation is associated with mutations in cytoplasmic carbonic anhydrase II. The metabolic alkalosis of congenital chloride-losing diarrhea is caused by mutations in the DRA Cl(-)/HCO3(-) exchanger of the ileocolonic apical membrane. Recessive osteopetrosis is caused by deficient osteoclast acid secretion across the ruffled border lacunar membrane, the result of mutations in the vH(+)-ATPase V0 subunit or in the CLC-7 Cl(-) channel. X-linked nephrolithiasis and engineered deficiencies in some other CLC Cl(-) channels are thought to represent defects of organellar acidification. Study of acid-base transport disease-associated mutations should enhance our understanding of protein structure-function relationships and their impact on the physiology of cell, tissue, and organism.

Impaired trafficking of distal renal tubular acidosis mutants of the human kidney anion exchanger kAE1

Distal renal tubular acidosis (dRTA) is an inherited disease characterized by the failure of the kidneys to appropriately acidify urine and is associated with mutations in the anion exchanger (AE)1 gene. The effect of the R589H dRTA mutation on the expression of the human erythroid AE1 and the truncated kidney form (kAE1) was examined in transfected human embryonic kidney 293 cells. AE1, AE1 R589H, and kAE1 were present at the cell surface, whereas kAE1 R589H was located primarily intracellularly as shown by immunofluorescence, cell surface biotinylation, N-glycosylation, and anion transport assays. Coexpression of kAE1 R589H reduced the cell surface expression of kAE1 and AE1 by a dominant-negative effect, due to heterodimer formation. The mutant AE1 and kAE1 bound to an inhibitor affinity resin, suggesting that they were not grossly misfolded. Other mutations at R589 also prevented the formation of the cell surface form of kAE1, indicating that this conserved arginine residue is important for proper trafficking. The R589H dRTA mutation creates a severe trafficking defect in kAE1 but not in erythroid AE1.

Distal renal tubular acidosis associated with isolated large vestibular aqueduct and sensorineural hearing loss

Distal renal tubular acidosis (dRTA) is characterized by a defect in urinary acidification with various degrees of metabolic acidosis; it can be inherited either as an autosomal dominant trait or as a recessive trait. The recessive form is associated in about one third of cases with progressive sensorineural hearing loss (SNHL). We performed a neuroradiological study in 3 consecutive unrelated pediatric patients affected with sporadic dRTA and progressive SNHL that disclosed an enlarged vestibular aqueduct (VA) and endolymphatic sac (ES) in each. The presence of an enlarged VA in our patients with dRTA and SNHL could contribute to the development, or at least the progression, of the hearing impairment. We suppose that the same molecular defect present in both the kidney and the inner ear could be the cause of dRTA and of the development of the enlarged VA and ES.

Band 3 anion exchanger and its involvement in erythrocyte and kidney disorders

Recent developments in the structure of erythrocyte band 3 and its role in hereditary spherocytosis and distal renal tubular acidosis are described. The crystal structure of the N-terminal cytoplasmic domain provides a basis for understanding the organization of ankyrin and other peripheral membrane proteins around band 3. Band 3 also binds integral membrane proteins, including the Rh protein complex and CD47. Band 4.2 is important in these associations, which link the Rh complex to the skeleton. It is suggested that band 3 forms the scaffold for a protein assembly that could transduce signals from the cell exterior and modulate the transport and mechanical properties of the erythrocyte. The involvement of band 3 in distal renal tubular acidosis is reviewed. The article discusses a likely mechanism for dominant distal renal tubular acidosis in which associations between the normal and mutant protein alter the plasma membrane targeting of the normal protein in the kidney.

Band 3 Walton, a C-terminal deletion associated with distal renal tubular acidosis, is expressed in the red cell membrane but retained internally in kidney cells

Human band 3 Walton is an AE1 mutation that results in the deletion of the 11 COOH-terminal amino acids of the protein and is associated with dominant distal renal tubular acidosis. The properties of band 3 Walton expressed with normal band 3 in the heterozygous mutant erythrocytes and the kidney isoform expressed in Xenopus oocytes and in the Madin-Darby canine kidney cell line were examined. The mutant erythrocytes have normal hematology but have reduced band 3 Walton content. Transport studies showed that erythrocyte band 3 Walton has normal sulfate transport activity, and kidney band 3 Walton has normal chloride transport activity when expressed in Xenopus oocytes. The mutant protein is clearly able to reach the cell surface of erythrocytes and oocytes. In contrast, while normal kidney band 3 was expressed at the cell surface in the kidney cell line, the Walton mutant protein was retained intracellularly within the kidney cells. The results demonstrate that band 3 Walton is targeted differently in erythrocytes and kidney cells and indicate that the COOH-terminal tail of band 3 is required to allow movement to the cell surface in kidney cells. It is proposed here that the mutant band 3 gives rise to dominant distal renal tubular acidosis by inhibiting the movement of normal band 3 to the cell surface. It is suggested that this results from the association of the normal and mutant proteins in band 3 hetero-oligomers, which causes the intracellular retention of normal band 3 with the mutant protein.

Physiological roles and regulation of mammalian sulfate transporters

All cells require inorganic sulfate for normal function. Sulfate is among the most important macronutrients in cells and is the fourth most abundant anion in human plasma (300 microM). Sulfate is the major sulfur source in many organisms, and because it is a hydrophilic anion that cannot passively cross the lipid bilayer of cell membranes, all cells require a mechanism for sulfate influx and efflux to ensure an optimal supply of sulfate in the body. The class of proteins involved in moving sulfate into or out of cells is called sulfate transporters. To date, numerous sulfate transporters have been identified in tissues and cells from many origins. These include the renal sulfate transporters NaSi-1 and sat-1, the ubiquitously expressed diastrophic dysplasia sulfate transporter DTDST, the intestinal sulfate transporter DRA that is linked to congenital chloride diarrhea, and the erythrocyte anion exchanger AE1. These transporters have only been isolated in the last 10-15 years, and their physiological roles and contributions to body sulfate homeostasis are just now beginning to be determined. This review focuses on the structural and functional properties of mammalian sulfate transporters and highlights some of regulatory mechanisms that control their expression in vivo, under normal physiological and pathophysiological states.

Renal tubular acidosis: a new look at an old problem

Although the definition of renal tubular acidosis (RTA) is simple, understanding the physiologic basis underlying the various types of this clinical entity is much more difficult. The pathophysiology of this disorder is reviewed using the normal acid-base functions of the involved segments of the nephron as a guide to understanding. Clinical and laboratory features of the subtypes of RTA are addressed, and diagnosis and treatment discussed. New developments in the knowledge and understanding of the associated growth disturbances, mineral metabolism, and molecular biology of RTA are also reviewed to provide the most current view of this relatively common pediatric entity.

A family-based study of metabolic phenotypes in calcium urolithiasis

BACKGROUND

A family history increases the risk of kidney stone passage independent of dietary risk factors. However, the metabolic basis for familial aggregation of urolithiasis is unknown.

METHODS

We evaluated metabolic risk factors in families with at least two sibs with a history of calcium stones. Sibs underwent outpatient evaluations simultaneously, including 24-hour urine collection and oral calcium loading. Phenotypes were compared between affected and unaffected sibs from the same sibship.

RESULTS

Eighty-three sibships comprising 388 sibs (212 affected sibs, 114 males and 98 females, and 176 unaffected sibs, 68 males and 108 females) from 71 families were analyzed. Daily urine calcium excretion was higher in affected compared with unaffected sibs (0.64 +/- 0.33 vs. 0.50 +/- 0.22 mmol Ca(2+)/mmol creatinine, respectively, P < 10(-5)). This corresponded to absolute values of 7.4 +/- 3.9 and 5.1 +/- 2.3 mmol Ca(2+)/day, respectively, for affected and unaffected males, and 5.4 +/- 2.6 and 4.2 +/- 1.9 mmol Ca(2+)/day, respectively, for affected and unaffected female sibs. When analyzed by tertile of onset age of stone passage, the differences in urine calcium were only significant in the first two tertiles (with onset age of stone passage <35 years). The fasting urine Ca(2+)/creatinine ratio was significantly higher in stone formers compared with control sibs (0.46 +/- 0.27 vs. 0.40 +/- 0.27, P = 0.04), as was the postcalcium load Ca(2+)/creatinine ratio (0.57 +/- 0.46 vs. 0.43 +/- 0.41, respectively, P = 0.02). Body mass index was marginally significantly higher in stone forming sibs (P = 0.04). Other urine phenotypes, including oxalate, phosphate, magnesium, citrate, urate, sodium, ammonium, and volume, were not associated with stone passage.

CONCLUSION

Increased urine calcium excretion is the only phenotype associated with a kidney stone formation in these French-Canadian families.

Hereditary distal renal tubular acidosis: new understandings

The primary or hereditary form of distal renal tubular acidosis (dRTA), although rare, has received increased attention recently because of dramatic advances in the understanding of its genetic basis. The final regulation of renal acid excretion is effected by various acid/base transporters localized in specialized cells in the cortical collecting and outer medullary collecting tubules. Inherited defects in two of the key acid/base transporters involved in distal acidification, as well as mutations in the cytosolic carbonic anhydrase gene, can cause dRTA. The syndrome is inherited in both autosomal dominant and recessive patterns; patients with recessive dRTA present with either acute illness or growth failure at a young age, sometimes accompanied by deafness, whereas dominant dRTA is usually a milder disease and involves no hearing loss. The AE1 gene encodes two Cl-/HCO3- exchangers that are expressed in the erythrocyte and in the acid-secreting intercalated cells of the kidney. AE1 contributes to urinary acidification by providing the major exit route for HCO3- across the basolateral membrane. Several mutations in the AE1 gene cosegregate with dominant dRTA. The modest degree of hypofunction exhibited in vitro by these mutations, however, does not explain the abnormal distal acidification phenotype. Other AE1 mutations have been linked to a recessive syndrome of dRTA and hemolytic anemia in which hypofunction can be discerned by in vitro studies. Several mutations in the carbonic anyhdrase II gene are associated with the autosomal recessive syndrome of osteopetrosis, renal tubular acidosis, and cerebral calcification. Some of these individuals present with deafness of the conductive type. By contrast, more recent studies have shown that mutations in ATP6B1, encoding the B-subtype unit of the apical H(+) ATPase, are responsible for a group of patients with autosomal recessive dRTA associated with sensorineural deafness. Thus, the presence of deafness and the type provide an important clue to the genetic lesion underlying hereditary dRTA.

Mapping of five new putative anion transporter genes in human and characterization of SLC26A6, a candidate gene for pancreatic anion exchanger

A second distinct family of anion transporters, in addition to the classical SLC4 (or AE) family, has recently been delineated. Members of the SLC26 family are structurally well conserved and can mediate the electroneutral exchange of Cl(-) for HCO(-)(3) across the plasma membrane of mammalian cells like members of the SLC4 family. Three human transporter proteins have been functionally characterized: SLC26A2 (DTDST), SLC26A3 (CLD or DRA), and SLC26A4 (PDS) can transport with different specificities the chloride, iodine, bicarbonate, oxalate, and hydroxyl anions, whereas SLC26A5 (prestin) was suggested to act as the motor protein of the cochlear outer hair cell. We report the expansion of the SLC26 family with five new members in chromosomes 3, 6, 8, 12, and 17 and mapping of SLC26A1 to 4p16.3. We have characterized one of them, SLC26A6, in more detail. It maps to chromosome 3p21.3, encodes a predicted 738-amino-acid transmembrane protein, and is most abundantly expressed in the kidney and pancreas. Pancreatic ductal cell lines Capan-1 and Capan-2 express SLC26A6, and immunohistochemistry localizes SLC26A6 protein to the apical surface of pancreatic ductal cells, suggesting it as a candidate for a luminal anion exchanger. The functional characterization of the novel members of this tissue-specific gene family may provide new insights into anion transport physiology in different parts of the body.

Inherited renal tubular acidosis

The past few years have witnessed great progress in elucidating the molecular basis of inherited renal tubular acidosis. Consistent with the physiologically defined importance of multiple gene products in urinary acidification, heritable renal tubular acidosis is genetically heterogeneous. Autosomal dominant distal renal tubular acidosis has been associated with a small number of mutations in the AE1 Cl-/HCO3- exchanger although the pathophysiologic mechanisms behind these mutations remain unclear. Rarely, autosomal recessive distal RTA is caused by homozygosity or compound heterozygosity for the loss-of-function mutation AE1 G701D. A larger proportion, often accompanied by hearing loss, is associated with mutations in the ATP6B1 gene encoding the 58 kDa B1 subunit of the vacuolar H+-ATPase. Mutations in the gene encoding the Na+/HCO3- cotransporter, NBC1, have recently been identified in proximal renal tubular acidosis with corneal calcification.

Band 3 mutations, renal tubular acidosis and South-East Asian ovalocytosis in Malaysia and Papua New Guinea: loss of up to 95% band 3 transport in red cells

We describe three mutations of the red-cell anion exchangerband 3 (AE1, SLC4A1) gene associated with distalrenal tubular acidosis (dRTA) in families from Malaysia and Papua NewGuinea: Gly(701)-->Asp (G701D), Ala(858)-->Asp(A858D) and deletion of Val(850) (DeltaV850). The mutationsA858D and DeltaV850 are novel; all three mutations seem to berestricted to South-East Asian populations. South-East Asianovalocytosis (SAO), resulting from the band 3 deletion of residues400-408, occurred in many of the families but did not itselfresult in dRTA. Compound heterozygotes of each of the dRTA mutationswith SAO all had dRTA, evidence of haemolytic anaemia and abnormal red-cell properties. The A858D mutation showed dominant inheritance and therecessive DeltaV850 and G701D mutations showed a pseudo-dominantphenotype when the transport-inactive SAO allele was also present. Red-cell and Xenopus oocyte expression studies showed that theDeltaV850 and A858D mutant proteins have greatly decreased aniontransport when present as compound heterozygotes (DeltaV850/A858D,DeltaV850/SAO or A858D/SAO). Red cells with A858D/SAO had only 3% ofthe SO(4)(2-) efflux of normal cells, thelowest anion transport activity so far reported for human red cells. The results suggest dRTA might arise by a different mechanism for eachmutation. We confirm that the G701D mutant protein has an absoluterequirement for glycophorin A for movement to the cell surface. Wesuggest that the dominant A858D mutant protein is possibly mis-targetedto an inappropriate plasma membrane domain in the renal tubular cell,and that the recessive DeltaV850 mutation might give dRTA because ofits decreased anion transport activity.

Evaluation of the calcium-sensing receptor gene in idiopathic hypercalciuria and calcium nephrolithiasis

BACKGROUND

Calcium urolithiasis is in part genetically determined and associated with idiopathic hypercalciuria.

METHODS

We have used a candidate gene approach to determine whether the calcium-sensing receptor (CaR) gene is linked to idiopathic hypercalciuria and calcium urolithiasis in a cohort of French Canadian sibships with multiple affected members (64 sibships from 55 pedigrees yielding 359 affected sibling pairs with > or =1 stone episode).

RESULTS

Using nonparametric linkage analysis with various intragenic and flanking markers, we showed that the CaR gene could be excluded as a major gene for hypercalciuric stone formation. We excluded the CaR (lod score <-2) at lambdas values of 1.5, 1.68, and 2.6 for sib pairs concordant for at least one stone passage, at least two stone passages, and at least one stone passage and calciuria above the 75th percentile, respectively. Quantitative trait linkage analyses did not suggest that the CaR gene was linked to biochemical markers of idiopathic hypercalciuria.

CONCLUSIONS

This study shows that genetic variants of the CaR gene are not associated with idiopathic hypercalciuria and calcium nephrolithiasis in this population of French Canadians.

Tubular proteinuria defined by a study of Dent's (CLCN5 mutation) and other tubular diseases

Tubular proteinuria defined by a study of Dent's ( CLCN5 mutation) and other tubular diseases.

BACKGROUND

The term "tubular proteinuria" is often used interchangeably with "low molecular weight proteinuria" (LMWP), although the former implies a definite etiology. A specific quantitative definition of tubular proteinuria is needed, and we address this by studying five different renal disorders.

METHODS

Tubular proteinuria was assessed by measuring urinary retinol-binding protein (RBP), beta2-microglobulin (beta2M), alpha1-microglobulin (alpha1M), and albumin in 138 patients: 26 affected males and 24 female carriers of the X-linked syndrome "Dent's disease," 6 patients with other Fanconi syndromes, 17 with distal renal tubular acidosis (dRTA), 39 with glomerulonephritis (GN), and 26 with Chinese herbs nephropathy (CHN).

RESULTS

RBP was better than beta2M or alpha1M in identifying the tubular proteinuria of Dent's disease. Median urinary RBP levels in mg/mmol creatinine were: affected male Dent's, 18.2, N = 26; carrier female Dent's, 0. 30, N = 24; dRTA, 0.027, N = 17; GN, 0.077, N = 39; and normal adults, 0.0079, N = 61. Elevated urinary RBP (>0.017) and albumin < (10 x RBP) + 2 identified all patients with the LMWP of Dent's disease and clearly distinguished their LMWP from that of dRTA and GN. This is a quantitative definition of tubular proteinuria. Consistent with this definition, 80% of those patients with CHN who had an elevated RBP had tubular proteinuria. Urinary RBP and albumin in carriers of Dent's disease were strikingly correlated over a 100-fold range (R = 0.933).

CONCLUSION

The combination of elevated urinary RBP (>0.017) and albumin < (10 x RBP) + 2 (mg protein/mmol creatinine) is a quantitative definition of tubular proteinuria. Furthermore, our findings suggest that a shared defect in tubular RBP and albumin reuptake causes this form of proteinuria.

Mammalian distal tubule: physiology, pathophysiology, and molecular anatomy

The distal tubule of the mammalian kidney, defined as the region between the macula densa and the collecting duct, is morphologically and functionally heterogeneous. This heterogeneity has stymied attempts to define functional properties of individual cell types and has led to controversy concerning mechanisms and regulation of ion transport. Recently, molecular techniques have been used to identify and localize ion transport pathways along the distal tubule and to identify human diseases that result from abnormal distal tubule function. Results of these studies have clarified the roles of individual distal cell types. They suggest that the basic molecular architecture of the distal nephron is surprisingly similar in mammalian species investigated to date. The results have also reemphasized the role played by the distal tubule in regulating urinary potassium excretion. They have clarified how both peptide and steroid hormones, including aldosterone and estrogen, regulate ion transport by distal convoluted tubule cells. Furthermore, they highlight the central role that the distal tubule plays in systemic calcium homeostasis. Disorders of distal nephron function, such as Gitelman's syndrome, nephrolithiasis, and adaptation to diuretic drug administration, emphasize the importance of this relatively short nephron segment to human physiology. This review integrates molecular and functional results to provide a contemporary picture of distal tubule function in mammals.

Erythroid band 3 variants and disease

This review describes some of the naturally occurring band 3 (AEI) variants and their association with disease. Southeast Asian Ovalocytic (SAO) band 3, an inactive and misfolded protein, is probably only maintained in certain populations because it provides protection against the cerebral form of malaria. Many mutations that cause instability of band 3, either at the mRNA or protein level, result in hereditary spherocytosis (HS). Some polymorphisms alter amino acid residues in the extracellular loops of band 3 and are associated with blood group antigens. A truncated form of AEI is expressed in kidney cells and certain AEI mutations are associated with distal renal tubular acidosis (dRTA). The molecular basis of these variants and their effect on the structure and function of band 3 are discussed. The association between band 3 and glycophorin A (GPA) and the structure/function changes of band 3 in the absence of GPA are also described.

Localization of a gene for autosomal recessive distal renal tubular acidosis with normal hearing (rdRTA2) to 7q33-34

Failure of distal nephrons to excrete excess acid results in the "distal renal tubular acidoses" (dRTA). Early childhood features of autosomal recessive dRTA include severe metabolic acidosis with inappropriately alkaline urine, poor growth, rickets, and renal calcification. Progressive bilateral sensorineural hearing loss (SNHL) is evident in approximately one-third of patients. We have recently identified mutations in ATP6B1, encoding the B-subunit of the collecting-duct apical proton pump, as a cause of recessive dRTA with SNHL. We now report the results of genetic analysis of 13 kindreds with recessive dRTA and normal hearing. Analysis of linkage and molecular examination of ATP6B1 indicated that mutation in ATP6B1 rarely, if ever, accounts for this phenotype, prompting a genomewide linkage search for loci underlying this trait. The results strongly supported linkage with locus heterogeneity to a segment of 7q33-34, yielding a maximum multipoint LOD score of 8.84 with 68% of kindreds linked. The LOD-3 support interval defines a 14-cM region flanked by D7S500 and D7S688. That 4 of these 13 kindreds do not support linkage to rdRTA2 and ATP6B1 implies the existence of at least one additional dRTA locus. These findings establish that genes causing recessive dRTA with normal and impaired hearing are different, and they identify, at 7q33-34, a new locus, rdRTA2, for recessive dRTA with normal hearing.

Autosomal recessive distal renal tubular acidosis associated with Southeast Asian ovalocytosis

BACKGROUND

A defect in the anion exchanger 1 (AE1) of the basolateral membrane of type A intercalated cells in the renal collecting duct may result in a failure to maintain a cell-to-lumen H+ gradient, leading to distal renal tubular acidosis (dRTA). Thus, dRTA may occur in Southeast Asian ovalocytosis (SAO), a common AE1 gene abnormality observed in Southeast Asia and Melanesia. Our study investigated whether or not this renal acidification defect exists in individuals with SAO.

METHODS

Short and three-day NH4Cl loading tests were performed in 20 individuals with SAO and in two subjects, including their families, with both SAO and dRTA. Mutations of AE1 gene in individuals with SAO and members of the two families were also studied.

RESULTS

Renal acidification in the 20 individuals with SAO and in the parents of the two families was normal. However, the two clinically affected individuals with SAO and dRTA had compound heterozygosity of 27 bp deletion in exon 11 and missense mutation G701D resulting from a CGG-->CAG substitution in exon 17 of the AE1 gene. Red cells of the two subjects with dRTA and SAO and the family members with SAO showed an approximate 40% reduction in sulfate influx with normal 4,4'-di-isothiocyanato-stilbene-2,2'-disulfonic acid sensitivity and pH dependence.

CONCLUSION

These findings suggest that compound heterozygosity of abnormal AE1 genes causes autosomal recessive dRTA in SAO.

Transmembrane folding of the human erythrocyte anion exchanger (AE1, Band 3) determined by scanning and insertional N-glycosylation mutagenesis

The human erythrocyte anion exchanger (AE1, Band 3) contains up to 14 transmembrane segments, with a single site of N-glycosylation at Asn642 in extracellular (EC) loop 4. Scanning and insertional N-glycosylation mutagenesis were used to determine the folding pattern of AE1 in the membrane. Full-length AE1, when expressed in transfected human embryonic kidney (HEK)-293 or COS-7 cells, retained a high-mannose oligosaccharide structure. Scanning N-glycosylation mutagenesis of EC loop 4 showed that N-glycosylation acceptor sites (Asn-Xaa-Ser/Thr) spaced 12 residues from the ends of adjacent transmembrane segments could be N-glycosylated. An acceptor site introduced at position 743 in intracellular (IC) loop 5 that could be N-glycosylated in a cell-free translation system was not N-glycosylated in transfected cells. Mutations designed to disrupt the folding of this loop enhanced the level of N-glycosylation at Asn743 in vitro. The results suggest that this loop might be transiently exposed to the lumen of the endoplasmic reticulum during biosynthesis but normally folds rapidly, precluding N-glycosylation. EC loop 4 insertions into positions 428, 484, 754 and 854 in EC loops 1, 2, 6 and 7 respectively were efficiently N-glycosylated, showing that these regions were extracellular. EC loop 4 insertions into positions 731 or 785 were poorly N-glycosylated, which was inconsistent with an extracellular disposition for these regions of AE1. Insertion of EC loop 4 into positions 599 and 820 in IC loops 3 and 6 respectively were not N-glycosylated in cells, which was consistent with a cytosolic disposition for these loops. Inhibitor-affinity chromatography with 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonate (SITS)-Affi-Gel was used to assess whether the AE1 mutants were in a native state. Mutants with insertions at positions 428, 484, 599, 731 and 785 showed impaired inhibitor binding, whereas insertions at positions 754, 820 and 854 retained binding. The results indicate that the folding of the C-terminal region of AE1 is more complex than originally proposed and that this region of the transporter might have a dynamic aspect.

Mutations in the gene encoding B1 subunit of H+-ATPase cause renal tubular acidosis with sensorineural deafness

H+-ATPases are ubiquitous in nature; V-ATPases pump protons against an electrochemical gradient, whereas F-ATPases reverse the process, synthesizing ATP. We demonstrate here that mutations in ATP6B1, encoding the B-subunit of the apical proton pump mediating distal nephron acid secretion, cause distal renal tubular acidosis, a condition characterized by impaired renal acid secretion resulting in metabolic acidosis. Patients with ATP6B1 mutations also have sensorineural hearing loss; consistent with this finding, we demonstrate expression of ATP6B1 in cochlea and endolymphatic sac. Our data, together with the known requirement for active proton secretion to maintain proper endolymph pH, implicate ATP6B1 in endolymph pH homeostasis and in normal auditory function. ATP6B1 is the first member of the H+-ATPase gene family in which mutations are shown to cause human disease.

Red blood cell membrane disorders

The recent discovery of the specific molecular defects in many patients with hereditary spherocytosis and hereditary elliptocytosis/pyropoikilocytosis partially clarifies the molecular pathology of these diseases. HE and HPP are caused by defects in the horizontal interactions that hold the membrane skeleton together, particularly the critical spectrin self-association reaction. Single gene defects cause red cells to elongate as they circulate, by a unknown mechanism, and are clinically harmless. The combination of two defective genes or one severe alpha spectrin defect and a thalassaemia-like defect in the opposite allele (alphaLELY) results in fragile cells that fragment into bizarre shapes in the circulation, with haemolysis and sometimes life-threatening anaemia. A few of the alpha spectrin defects are common, suggesting they provide an advantage against malaria or some other threat. HS, in contrast, is nearly always caused by family-specific private mutations. These involve the five proteins that link the membrane skeleton to the overlying lipid bilayer: alpha and beta spectrin, ankyrin, band 3 and protein 4.2. Somehow, perhaps through loss of the anchorage band 3 provides its lipid neighbours (Peters et al, 1996), microvesiculation of the membrane surface ensues, leading to spherocytosis, splenic sequestration and haemolysis. Future research will need to focus on how each type of defect causes its associated disease, how the spleen aggravates membrane skeleton defects (a process termed 'conditioning'), how defective red, cells are recognized and removed in the spleen, and why patients with similar or even identical defects can have different clinical severity. Emphasis also needs to be given to improving diagnostic tests, particularly for HS, and exploring new options for therapy, like partial splenectomy, which can ameliorate symptoms while better protecting patients from bacterial sepsis and red cell parasites, and perhaps from atherosclerosis (Robinette & Franmeni, 1977) and venous thrombosis (Stewart et al, 1996).

Novel AE1 mutations in recessive distal renal tubular acidosis. Loss-of-function is rescued by glycophorin A

The AE1 gene encodes band 3 Cl-/HCO3- exchangers that are expressed both in the erythrocyte and in the acid-secreting, type A intercalated cells of the kidney. Kidney AE1 contributes to urinary acidification by providing the major exit route for HCO3- across the basolateral membrane. Several AE1 mutations cosegregate with dominantly transmitted nonsyndromic renal tubular acidosis (dRTA). However, the modest degree of in vitro hypofunction exhibited by these dRTA-associated mutations fails to explain the disease phenotype in light of the normal urinary acidification associated with the complete loss-of-function exhibited by AE1 mutations linked to dominant spherocytosis. We report here novel AE1 mutations linked to a recessive syndrome of dRTA and hemolytic anemia in which red cell anion transport is normal. Both affected individuals were triply homozygous for two benign mutations M31T and K56E and for the loss-of-function mutation, G701D. AE1 G701D loss-of-function was accompanied by impaired trafficking to the Xenopus oocyte surface. Coexpression with AE1 G701D of the erythroid AE1 chaperonin, glycophorin A, rescued both AE1-mediated Cl- transport and AE1 surface expression in oocytes. The genetic and functional data both suggest that the homozygous AE1 G701D mutation causes recessively transmitted dRTA in this kindred with apparently normal erythroid anion transport.